Tuesday, 13 December 2016

An important source for human embryology, including implantation and formation of the placenta, is the Carnegie Collection now housed at the Human Developmental Anatomy Center in Washington D.C. The core of this collection is the carefully dated series of embryos first described by Hertig, Rock and Adams (here).The Virtual Human Embryo is an online ressource based on the serially sectioned embryos in this collection and includes 3D reconstructions. It covers all 23 Carnegie stages in the first 8 weeks of embryonic development and cannot be too highly recommended.

Now a group in Amsterdam has used the Carnegie Collection to develop an additional annotated digital atlas of human development (described here). They also utilized material from the Boyd Collection at the Centre for Trophoblast Research in Cambridge.They make two claims. First that representations in textbooks have become increasingly schematic. This is demonstrably true. Second that the descriptions in standard texts are often based on extrapolation to humans from animal models. It is hard to assess if the latter truly is the case. For example Human Embryology by Hamilton, Boyd and Mossman (previous post) was based on the human embryos in the possession of the three authors. In Germany there was a strong tradition to cover the embryology of all vertebrates, concluding with the human, exemplified by Dietrich Starck's Embryologie.In physiology, on the other hand, animal data often are presented as if they were human. One example concerns oxygen tensions in various parts of the fetal circulation. Pretty much every textbook of physiology has a large illustration of the fetal circulation with data obtained in sheep by Dawes, Mott and Widdicombe. The figure legends often fail to acknowledge the source or the species or both.

Wednesday, 26 October 2016

Viviparity and pregnancy has evolved numerous times and Camilla Whittington has set herself the ambitious task to seek similarities in gene expression that support pregnancy in sea horses, viviparous lizards and marsupial mammals. Her paper on the transcriptome of the brood pouch of a male sea horse was highlighted in a previous post.Now I am delighted to report that Camilla has been awarded a Fondation L'Oréal Women in Science Fellowship. It is good that this award exists and especially encouraging that research in comparative biology of pregnancy has become known to a wider public through the publicity surrounding Camilla's Award.

Sea horses from Camilla's web site

To learn more about Camilla's Work visit her web site. For hard science you can read about the comparative genomics of hormone signalling in the chorioallantoic membrane here.

There is much interest in how increased brain size impacted on dimensions of the pelvis, birth weight and placentation as humans evolved (here). Increased brain growth in fetal life would seem to demand a greater placental blood flow to ensure an adequate oxygen supply. This may have led to increased invasiveness (discussed here) and indirectly to the risk of preeclampsia (here). But what if there was an increase not only in size but in the metabolic rate of the brain?In a novel approach, Seymour, Bosiocic and Snelling attempted to assess cerebral blood flow in fossil adult hominins as a surrogate for oxygen supply to and consumption by the brain. As their starting point they measured the diameter of the carotid foramen in skulls of fossil hominins ranging from Australopithecus to archaic Homo sapiens (the carotid foramina carry the principal arteries supplying the brain). As can be seen in the figure, their main finding was that during hominin evolution cerebral blood flow increased disproportionately to brain size. The implication was that there was a progressive increase in the metabolic rate of the brain. Of course this approach required some major assumptions (see below). But if the metabolic rate of the adult brain did increase successively as humans involved, so perhaps did that of the fetal brain. Here more is at play than the rate of blood flow to the brain. In adults the blood becomes fully saturated in the lungs, but that is not the case in fetal life. The oxygen content of the blood reaching the brain is dependent on placental function (reviewed here).For Neanderthals there is enough data to conclude that brain size at birth did not differ from that of modern humans (here). But if human fetal brain had a higher metabolic rate than Neanderthal fetal brain, it might still have required a more efficient placenta.Assumptions: The approach adopted by Seymour et al. relied on rearranging an equation for shear stress to isolate one of its determinants, blood flow rate. To accomplish this, shear stress must first be estimated using a scaling model that relates shear stress to body size. This is the weak point in the analysis, but the authors point to a previous study that verified the approach in primates and marsupials

Tuesday, 4 October 2016

I thoroughly recommend this new book on South American fossil mammals of the Caenozoic Era. Intended for a broad readership, it is a times fanciful. The full caption to the above figure reads, "A giant caiman (Purussaurus neivensis) finally strikes in a fury of mud and water after quietly approaching a group of smalllitopterns (Megadolodus molariformis) drinking at the river."

Indiana University Press ISBN 978-0-253-02084-0

The book is dedicated "to anyone who has ever wondered what a notoungulate looked like," which makes me the target audience. I read extensively about the fossil fauna when writing about the placentation of the extant species (here).
The extinct families that lived side by side with marsupials and earlier xenarthrans were so unfamiliar as to seem wraith-like.

Darin A. Croft has worked closely with Velizar Simeonovski to bring extinct taxa to life. "The xenungulate Carodnia vieirai passes under the shadows of the dense vegetation of Itaboraí at high noon" is another example. This is the same artist who illustrated Extinct Madagascar (previous post).After some introductory chapters the subject matter is arranged according to 15 fossiliferous localities. Conspicuously absent is Lagoa Santa in Brazil where the Danish palaeontologist P. W. Lund worked in the Nineteenth Century (here). His extensive Collection is housed at the Natural History Museum of Copenhagen University.

Thursday, 22 September 2016

The Caribbean is home to two endemic families of insectivores. The Nesophontidae are recently extinct. Their demise is attributed to the introduction of rats by Spanish Explorers though some believe the genus survived into the 20th Century.To establish their phylogenetic position, scientists recently extracted DNA from a specimen preserved in an ancient owl pellet (here). This was no mean feat as the specimen was 750-years-old and DNA degrades rapidly in the tropics.
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The main finding was that Nesophontes shares a common ancestor with Solenodon. There are two species of this insectivore both listed as threatened by IUCN. A common origin had not been predicted on anatomical grounds not least because Nesophontes and Solenodon have different patterns of tooth occlusion (here). Together these families represent the oldest branch of the insectivores (Order Lipotyphla).

Placenta of Solenodon paradoxus Wislocki (1940)

Despite its rarity, the placentation of S. paradoxus has been described (here) and we later compared it with that of other insectivores (here). Interesting features are remnants of capsular decidua, elaborately branched yolk sac villi and a sheath that superficially resembles that of crociduran shrews.

Wednesday, 21 September 2016

The Invention of Nature: The Adventures of Alexander von Humboldt: The Lost Hero of Science by Andrea Wulf was just awarded the Royal Society Insight Investment Science Book Prize 2016. Previously it won the 2015 Costa Biography Award.
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ISBN-13: 978-1848548985

This is a fantastic book. While rooted in the late 18th Century, Andrea Wulf convinces us that Alexander van Humboldt's life and legacy are pertinent today. He started life as a mine inspector and was in his early thirties before realising his dream and travelling in South America where he showed extraordinary stamina and a capacity for accumulating huge amounts of data in all areas of science.

Charles Darwin had a set of Humboldt's books in his cramped cabin on the Beagle and they were heavily annotated. Humboldt met and corresponded with American presidents in post Revolutionary America and influenced the thinking of conservationists such as John Muir. It was his way of thinking about the interrelatedness of the environment that led Ernst Haeckel to coin the very word ecology (from the Greek for household).

Statue of Alexander von Humboldt in front ofHumboldt-Universität in Berlin

Alexander's older brother Wilhelm was a Prussian diplomat and minister of education. He it was who founded the university in Berlin that now bears the family name.

The subtitle The Lost Hero of Science suggests Alexander von Humboldt has been largely forgotten in the English-speaking world. Andrea Wulf's biography puts this to rights. ﻿﻿

Wednesday, 24 August 2016

My studies of placental evolution were stimulated by the large-scale molecular phylogenetic analyses of mammals that appeared 15 years ago (e.g. here). Especially riveting was the revelation of four superordinal clades, among them Afrotheria. An excellent review of the progress made in molecular phylogenetics and phylogenomics in the last 15 years recently appeared under the heading "Mammal madness" (by Foley, Springer and Teeling here). ﻿

Competing hypotheses on the root of the mammalian treeFrom Mess and Carter (here) based on Springer (Chapter here)

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Some of the uncertainties arising from the earlier work remain. Most striking is the failure unequivocally to root the tree. The three competing hypotheses shown above are still in play. While consensus is tipping in favour of Model A above, retroposon analysis provides similar support for all three.This is not all. There remains uncertainty about the branch order within Laurasiatheria, which includes bats, carnivores, pangolins and even- and odd-toed ungulates. Such uncertainties make it difficult to plot the evolution of fetal membranes and placentation. Thus the likelihood that the common ancestor of extant eutherians had an endotheliochorial placenta is greater under Model A than the other two hypotheses (shown here).

Concatenation and coalescence

Analysis of large data sets involves some fancy statistics. Foley et al. give a fair account of the pitfalls in concatenation, used in all the early papers, and coalescence. Coalescent methods require much more computer time and are not applicable to large data sets unless short cuts are taken. So far this has not worked out too well. An article just appeared in Cladistics (here), which seems to demand a "correction," in reality a retraction, of a paper in PNAS based on a coalescent approach. The authors doth protest too much, methinks. As Foley et al. point out, some pretty weird results appeared in the early days of DNA studies, too.

Morphology and molecules

The attitude of some researchers to morphological data could also be more generous. Attempts to integrate morphology and molecules, such as O'Leary et al. (previous post) do throw up some counterintuitive results. But there are examples where fossils have been useful in bolstering hypotheses based on molecular data - the position of whales is a case in point (previous post). Similarly, Afrotheria is not well supported by morphology, but we have published an apparent synapomorphy in the form of the allantoic sac (here).Moreover fossil calibrations are the key to solving another conflict, which concerns the timing of ordinal diversification of mammals. Deservedly this is given close attention in Foley et al.'s excellent review. The sort of fossils we are talking about are too old to yield ancient DNA so we have only morphology to go on. Foley et al. finish on an optimistic note and predict the next 20 years of phylogenetic research "should result in the resolution and dating of the mammal tree of life."

Monday, 15 August 2016

Used to be that tree shrews (Scandentia) were regarded as the closest relatives to primates as cogently argued by Wilfrid Le Gros Clark. Molecular phylogenetics have brought that into question with some proposing colugos or "flying lemurs" (Dermoptera) as a better alternative. But there have several competing hypotheses (see Martin). Choosing the right one has been difficult in the absence of a colugo genome.Now that has been rectified in a comprehensive study by Mason and colleagues (here) who sequenced the genome of a Sunda colugo and compared it with genomes from 21 mammals. The result clearly came out in favour of colugos as the closest relatives to primates, supported by 20 shared indels and 16 shared retrotransposons. There are many morphological similarities between tree shrews and colugos but the tree constructed by Mason et al. implies these are due to convergent evolution. Within Euarchontoglires they find tree shrews as the sister to Glires (rodents and lagomorphs).

Museomics and hidden biodiversity within colugos

Current reference works recognize no more than two species of colugo: Sunda Colugo (Galeopterus variegates) and the Phillipine Colugo (Cynocephalus volans). Mason et al. conclude that there may be as many as 6 Sundaic species and 2 Phillipine ones. They reached this conclusion by extracting DNA from museum specimens of known provenance. As an example, the Eastern and western populations on Borneo are highly divergent consistent with topographical features creating barriers to dispersal.Currently we are re-examining the placenta and fetal membranes of colugos. We too have had to rely on museum specimens such as those collected by A.A.W. Hubrecht.The importance of museum collections has been highlighted before in this blog. It is nice that scientists other than morphologists have discovered their value and coined the term museomics for studies of DNA from ancient specimens. Those in the current study ranged from 28 to 121 years old. The oldest specimen from the Raffles Museum of Biodiversity Research is contemporaneous with those collected by Hubrecht for his study of the fetal membranes and now housed at Museum für Naturkunde in Berlin.

Thursday, 11 August 2016

Marsupials and so-called placental mammals share a common ancestor yet pursue very different reproductive strategies. This led Ingmar Werneberg and colleagues to ask what reproduction was like in the common ancestor of marsupials and placentals (here).
﻿﻿﻿﻿They sampled data from the literature and examined specimens in the Hill and Hubrecht Collections at Museum für Naturkunde in Berlin. (In published sources they found examination of photographs and drawings more reliable than verbal descriptions.) There is a huge data set in the Supporting Information.
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It was estimated that the last common ancestor of placental mammals had a gestation of around 4 months and a litter of 3-5 young. The newborn were altricial and had closed eyes and almost naked skin at birth. The precocial lifestyle of hoofed mammals and the guinea pig is a derived feature (previous post). The newborn of marsupials are, of course, highly altricial.A reconstruction was attempted of the neonate of the therian ancestor. This suggested it was anatomically more like extant placentals than extant marsupials. In contrast, the preweaning period was very long compared to intrauterine gestation and thus infancy in this ancestor was more marsupial-like. The relative timing of eye opening was between that of placental and marsupial mammals. Gestation lasted 31 days in this hypothetical ancestor. It was reduced to 21 days in the marsupial lineage while increasing fourfold in the placental lineage.
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Friday, 10 June 2016

"Illustrations of the Gravid Uterus from the Renaissance through the Nineteenth Century," compiled by Lawrence D. Longo and Lawrence P. Reynolds.This book contains several iconic images and many that are less well known. Each with a text about the author, artist and engraver as well as an analysis of the influence of the book on contemporary science and midwifery.Great pains have been taken with reproduction of the images. No doubt many were taken from rare books in Larry Longo's own library. It is a pity he did not live to see the result in print (previous post).

Afterbirth of the sheep with four neat rows of cotyledonsFrom Girolamo Fabrizio De Formato Foetu 1604

There are plenty of images of the placenta including a few from animals. Anatomists who had dissected the gravid uteri of ruminants and dogs sometimes represented human placenta as cotyledonary or zonary in shape.I am enjoying this book. It is a pity that the publisher (Springer Nature) did not employ a copy editor. There are many more typos than might be expected in a work of such high quality.

Friday, 27 May 2016

A fresh phylogenetic analysis (here) based on sequence data from all living tenrecs and one otter shrew allows a re-evaluation of tenrec systematics.

The study confirmed the web-footed tenrec (Limnogale mergulus) is nested in the genus Microgale and should henceforth be referred to as M. mergulus.However, the authors also suggest resurrecting the generic name Nesogale for two species hitherto placed in Microgale. These are Dobson's shrew tenrec (N. dobsoni) and Talazaci's shrew tenrec (N. talazaci). Support for this included a 4-codon deletion shared only by these two species and a separate 9-codon deletion lacking in these species but found in the remaining Microgale. They concluded that this lineage had diverged from other shrew tenrecs in the Miocene.

Placentation

Villous area of the placenta of Dobson's shrewtenrec (Nesogale dobsoni) stained for cytokeratin (brown)

We included N. talazaci and N. dobsoni in our study of placentation in shrew tenrecs (here). As in Microgale and Oryzorictes there was both a central labyrinth and a more peripheral villous area. We did not notice any differences that would set Nesogale apart.

Friday, 13 May 2016

A system created for cultivating mouse blastocysts has been applied successfully to describe the development of the human embryo for 13 days after in vitro fertilization. This is a step towards opening the black box in our understanding of human embryology (reviewed here). Hitherto we have been confined to interpreting the histological sections of embryos in the Carnegie Collection.Papers by two groups were just published: Shahbazi et al. in Nature Cell Biology and Deglincerti et al. in Nature. They used appropriate molecular markers to identify epiblast, primitive endoderm (hypoblast) and trophectoderm. In addition they used cytokeratin 7 and human chorionic gonadotrophin as markers for cyto- and syncytiotrophoblast.
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Both groups showed the appearance of cavities corresponding to the amnion and primary yolk sac as known from studies in the rhesus macaque by Enders, Schlafke and Hendrickx. In the macaque, the yolk sac (at bottom in the figure) is outlined by visceral endoderm (beneath the epiblast) and the more squamous parietal endoderm. These tissues were identified by Shahbazi et al. in human embryos and shown to express the endoderm marker GATA6. Deglincerti et al. found the GATA6 signal was low in the parietal cells and that they expressed the trophectoderm marker CDX2. This is an interesting observation but hardly justifies them calling these cells "yolk sac trophectoderm." The term was criticized by Janet Rossant in the accompanying News and Views (here) and it must be hoped it does not gain currency. As in the macaque, amnion formation was by cavitation. This is nicely described by Shahbazi et al. Unfortunately they use the term pro-amnion, which is appropriate in the mouse but not in primates (contrasted here).Differentiation of trophectoderm into cytotrophoblast and multinucleated syncytiotrophoblast was confirmed with appearance of lacunae in the latter as appropriate for Carnegie Stage 5c.

Tuesday, 26 April 2016

Newborn and afterbirth of six-banded armadillo(Euphractus sexcinctus) from Chapman 1901

Some of the earliest studies of placenta in the United States were those
of Henry C. Chapman published in Proceedings of the Academy of Natural
Sciences of Philadelphia (available on JStor). His observations on the placenta
of a six-banded armadillo (Euphractus
sexcinctus) were published in 1901 and went unsurpassed for over a century.
Importantly, Chapman noted that polyembryony, known from the more widely
studied nine-banded armadillo (Dasypus
novemcinctus), did not occur in Euphractus.

Fetal membranes of a kangaroo (Macropus giganteus).Note the small allantois. From Chapman 1881

Chapman is notable for an early study of the fetal membranes of
the Eastern grey kangaroo (Macropus
giganteus). He noted that there was a large yolk sac but a relatively small
allantois that did not form a placenta.

Chapman got his armadillo and kangaroo specimens from the Philadelphia
Zoo, but his African bush elephant (Loxodonta
africana) placenta was from Cooper and Bailey’s London Circus. His was one
of several early descriptions of elephant placentation, including a paper by
Assheton (here), but there was then a hiatus
until the classical work by Amoroso and Perry in 1964 (here).
Based on the records of the elephant keeper, Chapman was able to estimate
gestation to 650-655 days.

Henry Cadwalader Chapman (1845-1910)

Chapman came from a prominent Philadelphia family. His grandmother was a
Biddle and her sister had married a Cadwalader, which may explain his middle
name. He studied medicine then spent three years in Europe under Richard Owen in
London and Alphonse Milne-Edwards in Paris.

Chapman’s work has been cited by Mossman, Amoroso, Wislocki
and Enders (here), but is in danger of being forgotten. When the
next paper on Euphractus appeared in
2012 (here), Chapman’s earlier contribution was
not acknowledged.

The yolk sac comprises a vascular portion (trilaminar omphalopleure) and a non-vascular portion (bilaminar omphalopleure). The sketch above shows trophoblast of the vascular yolk sac (troph) absorbing secretions from a uterine gland (gl ep). It is unclear whether there is exchange between the capillaries on the maternal (m cap) and fetal (foet cap) sides.

Thursday, 31 March 2016

Liang Bua, the cave where Homo floresiensis was found.This image was originally posted to Flickr by Rosino (CC-BY-SA-2.0).

When a new hominin was described from Flores in Indonesia (here), it was quickly dubbed "hobbit" with reference to its small stature (1.1 m) and relatively large feet. Even more sensational, however, was the date of 15 thousand years ago (kya) based on charcoal from the same deposit. The implication was that the "hobbit" (Homo floresiensis) overlapped in time with our own species.Yesterday my Twitter feed lit up with news of a paper (here) that drastically revised the age of these fossils to between 60-100 kya. Briefly there had been slippage of more recent material, including charcoal, into an eroded area where the bones were found. The new study used several methods to obtain the earlier date. Unlike in the original report, these included dating of the bones themselves.

Skull of Homo floresiensisThis image was originally posted to Flickr (CC-BY-SA-2.0).

The new dates match those of stone tools ranging from 50-190 kya. The most recent of those dates corresponds with the arrival of H. sapiens in the region, leading to conjecture that this caused the demise of the "hobbits."There is a new flurry of speculation about the origin of this species. One idea (discussed here) is that it is derived from Homo erectus (previous post) and its small stature is an example of Island Dwarfism (as seen in pygmy elephants from Flores and pygmy hippopotami from Madagascar).Flores lies East of the Wallace Line, a water barrier that separates the Southeast Asian and Australasian flora and fauna (previous post). It seems not to have presented a barrier to hominins. The enigmatic Denisovans also crossed the Wallace Line (previous post).

Sunday, 27 March 2016

Field guide or doorstopper? Just published as a Princeton Field Guide, this tome runs to 664 pages and weighs 1.3 kg. It aims to be, "The first comprehensive field guide to all 279 bovid species." Where did that total come from when Mammal Species of the World recognizes half the number?
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Johns Hopkins University Press 2011 ISBN-13: 978-1421400938

The answer is the above book on ungulates (hoofed mammals) with a revised taxonomy based on the authors' Phylogenetic Species Concept (PSC). A useful comparison between old and new species can be found at the ultimateungulate site. Ungulate Taxonomy was criticised by Frank E. Zachos and colleagues both in a letter to Nature (here) and in a detailed critique (here). They argued that splitting of species was a worrying trend with unfortunate implications for conservation efforts.

Bovid placentation

I bought both these books because bovid placentation deserves further study. Two major clades are recognized. Bovinae include domestic cattle (Bos taurus) and the zebu (B. indicus) - the latter was raised to species status by Groves and Grubb, which should please my Brazilian colleagues. Antilopinae is less well studied although it does include domestic sheep (Ovis aries). A large amount of antelope material is available in The Harland W. Mossman Collection together with detailed field notes by the principal collector Archie S. Mossman.

There is much to be done. And which of the rival terminologies should we use? I chose this rather fuzzy image of a klipspringer because Groves and Grubb chose to split it into no less than 11 species. Zachos et al. call this, "a prime example of rash taxonomic conclusions derived from inappropriate data." Even if their judgement is too harsh, the fact remains that this San Diego Zoo specimen cannot be assigned with confidence to any one of those 11 species. We might be on better ground with the Mossman material as it was collected in the wild at known localities and many of the "new" species have clearly defined (often restricted) ranges.

Hradecky wrote several papers on placentation in antelopes based mainly on the Mossman material and partly on specimens supplied by Benirschke (e.g. here). A set of his slides is in the Mossman Collection and some reproduced on the Benirschke web site.

Handbook of the Mammals of the World

PSC may be a valid taxonomic approach inasmuch as the premisses are defined and understood by experts. But there was renewed controversy when the Groves and Grubb taxonomy was incorporated in Volume 2 of Handbook of the Mammals of the World. Heller et al. (here) criticised PSC and concluded, "Conveying the message to the public that global diversity is on the decrease ... is unnecessarily confounded when the number of bovid species has just doubled without sufficient justification."

Perhaps the same criticism could be levelled at the new field guide. It does, however, have the virtue of supplying an illustrated version of Groves and Grubb - a book that was strangely lacking in pictures (as noted here). Trophy Hunters will find it a useful aid to bagging yet more species.

Wednesday, 23 March 2016

Museums house vast collections of specimens. For major museums of Natural History, these can be numbered in tens of millions. The specimens of greatest interest to embryologists will be in the wet collection and preserved as a rule in alcohol. Curating these specimens is an expensive business. When Chicago's Field Museum built a new Collections Resource Center the bill ran to $65 million and so stretched the budget that it resulted in cuts to museum staff (here).

Now museum collections in the US are under further threat as the National Science Foundation has suspended its funding for the maintenance of biological research Collections (here). This is a matter of great concern.Thanks to the Field Museum were able to describe the placenta from a rare specimen of an aquatic tenrec (above). We also used Field Museum specimens when reviewing the reproductive organs of bats (here).

Some museums house dedicated collections of embryological material. The J. P. Hill Collection is a good example and we have used it for several projects, including the first description of deep trophoblast invasion in a non-human primate (here).The history of this collection is instructive. After Hill's death in 1954 it was stored in the attic of Dixon Boyd's house in Cambridge. It subsequently found what seemed to be a permanent home at the Hubrecht Laboratory in Utrecht. Together with the Hubrecht Collection it was eventually kept in a purpose built facility of a new building. With a change of director this space was seen as more suitable for -70 freezers and the future of the collection remained uncertain until it was removed to Berlin. The director subsequently became President of the Royal Netherlands Academy of Arts and Sciences!

Tuesday, 8 March 2016

Not all bony fish are teleosts. Teleostei is an infraclass of ray-finned fish (Actinopterygii) with bichirs, sturgeons and gars (some 50 species) all basal to to the more numerous teleosts (some 30,000 species). The phylogeny of bony fishes has been largely resolved by Near et al. (open access).

Romer's Man and the Vertebrates did not make much of garpikes, but did include a great photo (shown above) of the largest species. My yellowing copy predates the genetic code yet alone genomics.What we now know is that teleosts are characterized by the Teleost Gene Duplication (TGD). This event was followed by rapid sequence evolution that may explain their success as a group. It can, however, make it difficult to identify orthologs to tetrapod genes and frustrate interpretation of data obtained in zebra fish (Danio rerio). To provide a bridge, Braasch et al. have sequenced and have just published the genome of the spotted gar (open access).To my knowledge no living species of the gar family is viviparous.

As in the Virginia opossum, the yolk sac is comprised of avascular and vascular portions. The avascular part follows the contours of the uterine epithelium (at left in the figure). The vascular portion attaches to it (at right).

The study of Philander by Enders and Enders was the first to show invasive trophoblast in a marsupial. In the figure there is a fold of uterine wall with intact epithelium at left and right, but this has disappeared at the centre of the fold, which is occupied by large trophoblast cells. Subsequently invasive trophoblast has been described in another opossum (Monodelphis domestica) and in an Australian marsupial (Sminthopsis crassicaudata).

Photomontage of a section through one uterus of Philander opossum.The two fetuses share a common yolk sac, but each has its ownallantoic sac one of which can be seen.Courtesy of Dr. Allen C. Enders

An unusual feature in Philander is that two or more fetuses share a common yolk sac. However, each has its own allantoic sac. The allantois never makes contact with the trophoblast (it remains within the small exocoelom). It may serve as a receptacle for urine secreted by the mesonephros.

Friday, 26 February 2016

Embryo of Virginia Opossum (Didelphys virginiana) at 6 days of gestation. Opened to show the vessels of the trilaminar yolk sacand free-floating allantoic sac. From Selenka 1886.

Emil Selenka did not confine himself to rodents and primates (previous post and recent review). He also gave the first detailed description of embryonic development in a marsupial, the Virginia opossum. He bred them in the laboratory and thus had dated pregnancies.All marsupials have a yolk sac placenta with both two-layered and three-layered areas (bilaminar and trilaminar omphalopleure); the latter has blood vessels that radiate from a sinus terminalis as can be seen in Selenka's illustration.
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Neonate of Virginia Opossum. From Selenka 1886.

﻿The Virginia opossum has a pretty short gestation even for a marsupial. The neonate above was born after 13 days. Development is supported largely by histotrophic nutrition, i.e. uptake of uterine gland secretions. The vascular part of the yolk sac is thought to be more important for gaseous exchange.Note that the allantois makes no contact with the trophoblast. Unlike in the koala, wombat and bandicoots, there is nothing approaching chorioallantoic placentation. The allantois serves mainly as a receptacle for urine excreted by the mesonephros (here).

There are 87 species of didelphids in Central and South America, but the Virginia opossum is alone in extending its range to the USA and Canada. Despite the species richness, placentation has been described for only five opossums; three from the same genus (D. virginiana, D. aurita and D. marsupialis) plus the gray short-tailed opossum (Monodelphis domestica) and gray four-eyed opossum (Philander opossum). I will save them for a later post.